1. Field of the Invention
The present invention relates to a data transmission method between a base station controller (BSC) and a base transceiver station (BTS) in a digital mobile communication system (such as a digital cellular system, a personal communication system, and a digital trunked radio system) employing a code division multiple access (CDMA) mode.
2. Description of Related Art
Generally, the digital mobile communication system applies to mobile objects such as a person, automobile, ship, train, and airplane.
FIG. 1 is a block diagram showing such conventional digital mobile communication system.
As shown in FIG. 1, the conventional digital mobile communication system comprises: mobile station (MS) 10 carried by a subscriber for wireless telecommunication; base transceiver station (BTS) 20 for receiving a call processing request from the MS 10 and transmitting a call transmission request from base station controller (BSC) 30 to the MS 10; BSC 30 for controlling the BTS 20 to perform the signal processing between the BTS 20 and mobile switching center (MSC) 40; and MSC 40 connected to the BSC 30 for transmitting the call processing request from the MS 10 via a public network or a private network to a public switching telephone network (PSTN) or advanced mobile phone service (AMPS) and another communication network to offer mobile communication service.
In this conventional mobile communication system, if a subscriber having his/her MS 10 attempts to use the mobile communication service within a service radius of the MSC 40, the MSC 40 catches the location of the MS 10 and operates to perform voice/facsimile information service according to the request of the MS 10 or connects the MS to another communication network to perform the mobile communication service.
FIG. 2 is a block diagram showing data transmission between the BSC 30 and the BTS 20.
As shown in FIG. 2, the BSC 30 comprises: vocoder 31 for compressing voice or data received from the MSC 40 to generate packets and transmitting voice or data received from the BTS 20 to the MSC 40; selection unit 32 for generating frames from the voice or data packets received from the vocoder 31 and analyzing frames received from the BTS 20; global positioning system (GPS) receiver 33 for receiving time/frequency from a GPS satellite and generating reference clocks for synchronization of the digital mobile communication system; and E1/T1 interface 34 for transmitting the frames received from the selection unit 32 via a trunk to the BTS 20 in synchronization with the reference clocks generated from the GPS receiver 33 and receiving the frames from the BTS 20.
The BTS 20 comprises: base station common processor (BCP) 21 for controlling frame transmission between the MS 10 and BSC 30, generating frames from voice or data received from the MS 10, and analyzing frames received from the BSC 30; channel card interface 22 corresponding to the vocoder 31 of the BSC 30 one to one, for modulating the frames received via an E1/T1 trunk from the BSC 30 according to control of the BCP 21 and demodulating voice or data received from the MS 10; GPS receiver 23 for receiving time/frequency from a GPS satellite and generating reference clocks for synchronization of the digital mobile communication system; and E1/T1 interface 24 for transmitting the frames generated from the BCP 21 via the E1/T1 trunk to the BSC 30 in synchronization with the reference clocks generated from the GPS receiver 23 and receiving the frames from the BSC 30 via the E1/T1 trunk.
The following description concerns how a high level data link control (HDLC) frame is transmitted via the E1/T1 trunk between the BSC 30 and BTS 20.
Primarily, transmission of frames from the BSC 30 to the BTS 20 is performed as follows. Once voice or data is transmitted from the MSC 40, the vocoder 31 in the BSC 30 receives the voice or data and compresses a transmission band to provide an output to the selection unit 32. At this time, the vocoder 31 transmits the voice or data to the selection unit 32 at a full rate if an amount of voice or data is received from the MSC 40. The vocoder 31 controls packet transmission according to a transmission rate of the voice or data to be transmitted to the selection unit 32. For example, if the transmission rate of the voice or data to be transmitted to the selection unit 32 is of the order of half the full rate, the vocoder 31 transmits a voice or data packet to the selection unit 32 at a half rate. In such manner, the voice or data is transmitted from the MSC 40 to the selection unit 32 via the vocoder 31 at different transmission rates, such as the full rate and transmission rates 1/2, 1/4, and 1/8, depending upon circumstances.
When the voice or data packet is transmitted from the vocoder 31 at one of the different transmission rates, the selection unit 32 writes the voice or data packet onto a corresponding field in an HDLC frame to generate the frame. The selection unit 32 writes the received packet onto a voice/data field having a fixed length of 20 bytes regardless of the transmission rate of the voice or data packet and writes onto header and tail fields in the frame to generate the frame having a fixed length of 48 bytes. The generated frame of the fixed length is transmitted to the E1/T1 interface 34. The E1/T1 interface 34 transmits the frame via the E1/T1 trunk to the BTS 20 according to the reference clock output from the GPS receiver 33.
The same bandwidth is used when transmitting packets from the vocoder 31 to the BTS 20 at a low transmission rate such as the transmission rate 1/8 and at the full rate. This results in waste of bandwidth in case of data transmission at the low rate such as the transmission rate 1/8.
In other words, the conventional data transmission method between the BSC 30 and the BTS 20 using the E1/T1 trunk is a fixed rate non-channelized HDLC method, wherein the bandwidth of 2.048 Mbps used in case of the E1 and the bandwidth of 1.544 Mbps used in case of the T1 are shaped into the fixed length transmission frame of 48 bytes regardless of channels.
The E1 trunk has transmission capacity to transmit one frame per 20 msec, thus transmitting 2400 bytes (48*50) per 1 second in the 48-byte fixed length frame transmission. Therefore, the data transmission via the E1 trunk has the bandwidth of 19.2 kbps (2400*8) and the capacity of the trunk is 2048 kbps/19.2 kbps=106. Such transmission capacity of the E1 trunk is effected when the vocoder of 8 kbps is applied.
Once the fixed length frame from the BSC 30 is received by the E1/T1 interface 24 in the BTS 20 via the E1/T1 trunk, the base station common processor 21 analyzes the received fixed length frame. The channel card interface 22 modulates the analyzed data before transmitting it to the MS 10.
Next, the description on transmission of voice or data of the MS 10 from the BTS 20 to the BSC 30 in the 48-byte fixed length frame will be set forth. The 48-byte frame is generated at the base station common processor 21 in the BTS 20 and transmitted via the E1/T1 trunk to the BSC 30. The selection unit 32 in the BSC 30 then receives and analyzes the frame and transmits the analyzed voice or data to the vocoder 31 for transmission to the MSC 40.
In this case of transmitting the data from the BTS 20 to the BSC 30, the data is also transmitted in the form of the 48-byte fixed length frame.
As illustrated above, the conventional method of transmitting the data between the BSC and BTS in the mobile communication system using the E1/T1 trunk transmits the data in the form of a frame of the same length both when the voice or data is transmitted at the full rate and when the voice or data is transmitted at the low rate such as the transmission rate 1/8. Therefore, there is a problem of wasting the bandwidth when transmitting the data at other rates than the full rate.
Such waste of the bandwidth of the trunk results in lowering of transmission efficiency.
Additionally, when applying the vocoder of 13 kbps in association with the vocoder of 8 kbps, voice or data packets to be actually transmitted increase and the capacity of the trunk decreases in proportion to the packet increase, so it is required to additionally install a special trunk.